The present invention relates to fault protection for a medium voltage transformer branches equipped with current interrupters for switching operating currents and for protecting the branch against fault currents.
In most cases, fault protection for medium voltages, e.g. voltages between 5 and 40 kV, is effected by means of a combination of on-load switches with fuses (so-called HH fuses), with the on-load switches taking over the switching in and out of operating currents while the fuses are provided to protect the transformers against excess currents of all types, particularly short-circuit currents.
The use of fuse elements gives rise to serious drawbacks which can be subdivided into thermal problems and insulation problems.
(a) Thermal Problems:
Up to a multiple of the rated fuse current, no reliable current interruption takes place. However, in this current range, the fuses are thermally overloaded and they may burst, resulting in subsequent interfering arcs in the switching systems, or become a source of combustion for the surrounding insulating members.
Such thermal stresses also result in spreading of the fuse characteristics. Changes in the characteristics are also produced by lighting strikes which do not immediately cause the fuse to be blown and by frequent switch-on processes. The consequence of such changes in characteristics in three-phase faults may be single-phase fuse interruptions. During the subsequent opening of the on-load switch, the two other phases must be turned off by the on-load switch. However, many of the introduced on-load switch principles are not suitable for such fault current interruptions which usually coincide with a low cos .phi..
(b) Insulation Problems:
The fuses customarily installed in air or air-solid insulations are often the cause of malfunctions primarily in small, not air-conditioned network stations. Therefore, attempts have been made in the past to install so-called I.sub.k interrupters in oil-filled transformers. Details of these problems are described in "Fabrikfertige, typgeprufte, vollisolierte Kompaktstationen mit Transformatoren and I.sub.k -Unterbrechern" [Factory Ready, Type Tested, Fully Insulated Compact Stations Including Transformers and I.sub.k Interrupters], Elektrizitatswirtschaft [Electrical Economics], Volume 83 (1984) No. 7, pages 318-321.
However, the transformers employed for this purpose are special structures and are not acceptable to a number of customers. These solutions cannot be used at all by customers in many countries.
To improve the problem of availability of the switching systems, fully encapsulated, low-maintenance on-load switching systems, e.g. those insulated with SF.sub.6, have been developed in recent years. However, for maintenance reasons, the fuse elements employed for the transformer branch are installed in separate compartments of the switch gear, usually insulated by air or solid insulating materials. This results in a system specific insulation whose problems are described, inter alia, in "Schaltanlage fur Ortsnetzverteiler-Stationen mit Vakuumleistungsschaltern" [Local Mains Distribution Stations Employing Vacuum Power Switches], Elektrotechnische Zeitschrift [Electrical Magazine], Volume 106 (1985), No. 10. A further drawback of this concept is the fact that the separate room for the fuses increases the dimensions of the system, as indicated, for example, in German Pat. No. 3,107,911 and counterpart U.S. Pat. No. 4,523,253.
The stated drawbacks are overcome in a prior art SF.sub.6 insulated switching system for local network exchange stations in which vacuum power switches are used in the transformer branch instead of the combination of on-load switches with fuses. This switching system is described in greater detail in the last paragraph of the above-cited paper. The vacuum power switch then interrupts all operating and fault currents that may be generated in the transformer due to interference and also in the primary input line. However, due to the high costs for the vacuum power switches, this solution must be considered uneconomical for use in network stations.
The problem behavior of high voltage, high power fuses is also referred to in FRG-OS No. 2,315,496, which proposes to solve these problems with an electronic circuit which causes actuation of the on-load switch in the critical current range of the fuses and thus prevents their being overloaded.
However, the protection against greater interference currents and short-circuit currents is left up to the fuses so that all disadvantageous insulation problems and questions of switching system dimensions remain in existence in this solution as well.